Conventional mounting arrangements for large mirror call for flexured tangent bars to be mounted at spaced intervals on the periphery of the mirror. A typical such system has three tangent bars, tangently mounted, at 120.degree. intervals about the periphery of the mirror. The tangent bars are in turn mounted to a cell or frame. Each of the tangent bars has one or more flexures integral thereto so as to allow the mirror to move out of its plane. Such movement is required to allow for axial and tilt adjustments of the mirror after assembly.
While movement of the mirror out of its plane is desirable, movement of the mirror in its plane is not. Accordingly, the mirror assembly, including tangent bars, is designed to have a very high resonance frequency. In this way, after the aforementioned adjustments are made, the mirror is held in a vibration-free state.
Conventional tangent bars, having flexures, have proved inappropriate for mounting large, heavy mirrors. This is so since the flexures required to achieve the desired high resonance are very stiff in bending. While they achieve the desired high resonance, these flexured tangent bars produce excessive distortion of the mirror surface when the axial location and tilt of the mirror are adjusted. Such distortions can also be introduced when the mirror and its associated housing, having different coefficients of thermal expansion, go through temperature excursions.
The present invention eliminates or ameliorates the foregoing disadvantages of the prior art by providing a tangent bar mount for large, heavy mirrors which has a high resonance frequency and introduces a minimum of distortion to the mirror when the axial location and tilt of the mirror is adjusted and when temperature excursions are experienced.
In addition, a weight compensation scheme is provided to eliminate surface figure distortions of the mirror due to the self-weight of the mirror.